A typical nuclear steam generator comprises a vertically-oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tubesheet for supporting the tubes at the ends opposite the U-shaped curvature, a dividing plate that cooperates with the tubesheet forming a primary fluid inlet plenum at the one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle, a primary fluid inlet nozzle in fluid communication with the primary fluid inlet plenum, and a primary fluid outlet nozzle in fluid communication with the primary fluid outlet plenum. The steam generator also comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber adjacent the shell and a feedwater ring disposed above the U-shaped curvature end of the tube bundle. The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet plenum, through the U-tube bundle, out the primary fluid outlet plenum, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator through the feedwater ring. The feedwater is conducted down the annular chamber adjacent the shell until the tubesheet near the bottom of the annular chamber causes the feedwater to reverse direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the feedwater is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the feedwater surrounding the tubes causing a portion of the feedwater to be converted to steam. The steam then rises and is circulated through typical electrical generating equipment thereby generating electricity in a manner well known in the art.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the U-tube walls which may be constructed by Inconel.RTM., the U-tube walls form part of the primary boundary for isolating these radioactive particles. It is, therefore, important that the U-tubes be maintained defect-free so that no breaks will occur in the U-tubes. However, experience has shown that under certain circumstances, the U-tubes may develop leaks therein which allow radioactive particles to contaminate the feedwater, which is a highly undesirable and dangerous result.
There is now thought to be at least two causes of tube leaks in steam generators. One cause of these leaks is considered to be related to the chemical environment of the feedwater side of the tubes. Analysis of the tube samples taken from operating steam generators which have experienced leaks has shown that the leaks were caused by cracks in the tubes resulting from intergranular corrosion. High caustic levels found in the vicinity of the cracks in the tube specimens taken from operating steam generators and the similarity of these cracks to failures produced by caustic under controlled laboratory conditions have identified high caustic levels as the cause of the intergranular corrosion and thus the cause of the tube cracking.
The other cause of tube leaks is thought to be tube thinning. Eddy current tests of the tubes have indicated that the thinning occurs on tubes near the tubesheet at levels corresponding to the levels of sludge that accumulates on the tubesheet. The sludge is mainly from oxides and copper compounds along with traces of other metals that have settled out of the feedwater onto the tubesheet. The level of sludge accumulation may be inferred by eddy current testing with a low frequency signal that is sensitive to the magnetic material in the sludge. The correlation between sludge levels and the tube wall thinning location strongly suggests that the sludge deposits provide a site for concentration of a phosphate solution or other corrosive agents at the tube wall that results in tube thinning.
One method for removing sludge from a steam generator is described in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System", issued Mar. 21, 1978 to Hickman et al. and assigned to the assignee of the present invention. In many nuclear steam generators in service today, there are six-inch diameter hand holes in the shell of the steam generator near the tubesheet that provides access to the tubesheet for removal of the sludge deposits on the tubesheet. With the system of Hickman et al., a fluid flushing stream is continuously maintained from a pair of flushing fluid injection nozzles inserted in one of the hand holes of the steam generator, around the annular space between the lower shell of the steam generator and the tube bundle, to a flushing fluid suction apparatus located at a second hand hole which diametrically opposes the first hand hole. While the fluid flushing stream is continuously maintained, a movable fluid lance is placed in the steam generator and moved along the tube lane to dislodge deposits from between the two brews and move the sludge toward and into the annular space where it is entrained in the continuously flowing flushing fluid stream. U.S. Pat. No. 4,276,856 issued to Dent et al. discloses a sludge lance advancing device used in carrying out the above-described process. However, often the pressure blast out of the high-pressure water jet is obstructed by other components of the sludge lance system.
U.S. Pat. No. 4,445,465 discloses a sludge lancing system which alternately directs the entire fluid flow first to the single movable lance for dislodging the sludge from between the tube rail while moving the sludge lance outwardly to the periphery of the tube bundle and then a stationary flushing fluid injector which directs the entirety of the available fluid about the periphery of the tube to flush the sludge which was previously dislodged by the movable lance toward a suction system. However, again, as with the previously discussed device, the pressure blast of the high-pressure water jet may be obstructed by various components of the sludge lance system. Consequently, the pressure blast of the high-pressure water jet may be diminished, thereby resulting in an insufficient amount of pressure to dislodge the settled sludge material. Various additional nuclear steam generator sludge lancing systems have been developed such as those disclosed in U.S. Pat. Nos. 4,715,324 issued to Muller et al. and 4,844,021 issued to Stoss; however, again, as with the above-mentioned prior devices, the pressure blast of the high-pressure water jet may become obstructed by various components of the sludge lance system resulting in the insufficient dislodging of the sludge material.
Therefore, there is clearly a need for a sludge lancing system which overcomes the aforementioned deficiencies found in the prior devices. More particularly, there is a need for a sludge lancing system wherein the half of the pressure blast generated by the high-pressure water jet is unobstructed by any component of the sludge lance system.